WO1992018750A1

WO1992018750A1 - Seabed gas collection and monitoring device

Info

Publication number: WO1992018750A1
Application number: PCT/GB1992/000628
Authority: WO
Inventors: Gary Young; Andrew Brown
Original assignee: Exal Reservoir Services Limited
Priority date: 1991-04-18
Filing date: 1992-04-08
Publication date: 1992-10-29
Also published as: GB9108351D0

Abstract

A seabed gas collection and monitoring device (10) for use on a seabed in the region of entry of a borehole being formed downwardly from the seabed by a drill and operating in conjunction with a permanent guide base (11) on the seabed which guides a rotating drill pipe and has upwardly extending guides (12) and guidelines (13) running between the surface and the guides (12) in order to guide the movement of equipment between a surface installation e.g. a semi-submersible drilling rig and guide base (11), in which the device (10) comprises: a downwardly open gas-collecting housing (14) to be positioned above the entrance to the borehole in order to collect any gas which issues upwardly from the borehole, a hole (15) arranged in housing (14) to allow a drill pipe to pass downwardly through the housing, a mounting frame (16) carrying housing (14) and which can be mounted on base frame guides (12) to mount the housing (14) above the borehole, an outlet fitting (18) provided on the housing (14) and arranged to direct any gas collected in the housing (14) laterally outwardly away from the borehole, and a remote electrically operated gas detector device (19, 39) to monitor receipt of gas by the housing (14) generated during drilling, and which passes to outlet fitting (18) and without being influenced by the rotating drill pipe.

Description

SEABED GAS COLLECTION AND MONITORING DEVICE

This invention relates to a seabed gas collection and monitoring device which is operable to monitor the presence of gas in a borehole during drilling downwardly from the seabed .

In the extraction of hydrocarbons from reservoirs below the seabed e.g. in the North Sea. it is usual to carry out the initial stages of drilling of the borehole downwardly from the seabed i.e. the drilling of the "upper hole sections", without use of a usual marine riser. The operation is controlled from a floating drilling rig (a semi -submersible or a drill ship) , and therefore the initial drilled formation returns or cuttings (including any hydrocarbons) only rise up the borehole to the seabed where they accumulate, rather than being conveyed to the rig where they could be monitored.

The main advantage of this existing procedure is that, since there is no direct connection between the seabed and the rig. if a shallow gas pocket is encountered during this initial stage of drilling, the rig can be moved laterally at the surface by pulling on its mooring lines until it reaches a safe area. However, this requires very close attention being given throughout the drilling operation to detect, at the earliest possible time, the issue of any potentially large volumes of gas. so that the rig can be moved away before such gas reaches the surface in any appreciable volume.

The issue of large volumes of gas is potentially very hazardous from two points of view. First of all. there is the risk of fire if a rig becomes engulfed by a large bubble of gas rising to the surface and passing upwardly around the rig. Secondly, the eruption of a large bubble of gas at the surface can create instantaneously a large "hole", and if this is created around or near to the rig. this can cause potentially hazardous conditions affecting the stability of the ri .

The detection of possible issue of potentially hazardous volumes of gas can be carried out purely by visual monitoring e.g. by observation of the surface of the usual "moonpool" on a drill ship, but this relies upon the diligence of anyone entrusted with the task, and whose duties can be made particularly difficult at night and / or during rough weather.

Evidently, by relying upon purely visual observations at the surface, very little time is available in which to take any remedial action, as the first indication of a possible problem will be by way of the first gas bubbles appearing at the surface.

Therefore, in order to give earlier warning of a potentially serious gas escape, and to provide an automatically operating system which does not rely upon human attention, it has been proposed to provide gas detection devices at the seabed in close proximity to the drill pipe and which are intended to detect the issue of gas and to trigger automatic warning devices on the rig at the surface.

However, in the design of a gas monitoring device for such use. there is a serious technical problem to be overcome, in that the device necessarily will detect and give warning of the presence of gas by electrical means e.g. by the detection of a variation in electrical resistance across a probe when gas is present, and the electrical signals produced during detection, and also during initiation of an alarm system, may be adversely influenced by the inevitable presence of substantial electrical interference or "noise" created in the immediate region of the rotating drill pipe. This can give rise to spurious signals when there is no gas hazard, or alternatively may mask detection of a truly potentially hazardous gas emission.

The present invention has therefore been developed primarily with a view to provide a more reliable gas monitoring device which is capable of being used on the seabed and of providing automatic monitoring of gas levels (and issue of warning signals when appropriate), despite interference created in the region of the rotating drill p i pe .

According to the invention there is provided a gas collecting and monitoring device which is intended to be used on a seabed in the region of entry of a borehole being formed downwardly from the seabed by a drill, there being a guide base on the seabed around the borehole and serving to guide the rotating drill, and which guide base has upwardly extending guides and guidelines running between the surface and the guides in order to guide the movement of equipment between the surface and the guide base, and said gas collecting and monitoring device comprising: a downwardly open gas-collecting housing which is intended to be positioned above the entrance to the borehole, and to collect any gas which issues upwardly from the borehole: a hole arranged in the housing to allow a drill pipe to pass downwardly through the housing; a mounting frame carrying the housing and arranged to come into engagement with the base frame guides in order to mount the housing above the borehole; an outlet fitting provided on the housing and arranged to direct any gas collected in the housing laterally outwardly away from the borehole; and. an electrically operated gas detector device arranged for monitoring the presence of gas in the outlet fitting, the detector device being arranged in use at a location sufficiently remote from the drill pipe that it can operate reliably to monitor the presence of gas without being influenced by the drill pipe.

Conveniently, the gas detector device has a gas sampling probe arranged in an outlet pipe of the outlet fitting, and which responds to a change in electrical resistance across the probe caused by the presence of gas, in order to detect the presence and the level of gas.

The use of a housing to collect the gas provides a relatively large collecting volume above the borehole, and which is then sampled via the relatively smaller volume outlet fitting, which effectively concentrates the level of gas and thereby enables early warning to be given of any potentially hazardous build-up of gas as drilling continues. Thus, during drilling, it would normally be anticipated that gas levels will steadily build-up in advance of actual entry of the drill head into a gas pocket.

However, it is not essential for the gas detector device to be mounted in the outlet pipe, and it may be arranged at a suitable location e.g. at the surface on the rig, provided that the gas sample passing along the outlet fitting is conveyed to the rig to be monitored by the gas detection device.

In a particularly preferred development of the invention, the problem of seabed deposit of drill cuttings is addressed, in that with existing techniques of upper hole drilling without a marine riser, it is not possible to carry out regular monitoring of the cuttings to extract necessary geological data as drilling progresses.

Therefore, in a preferred development of the invention, the outlet fitting of the housing leads to a jet pump which is operable to pump any gas to the rig via a suitable line, and which thereby creates a sufficient pressure drop within the housing to pull-in a sample of drilled formation cut.tings which is also passed up the line with the gas to the rig for routine analysis.

On the rig. the gas and the cuttings can then be separated, and the gas may be removed by a small de-gasser and be analysed by the sensitive gas chromatograph of a mud logging unit which is connected to the rig alarm system. Cuttings are removed by means of a sieve and can then be physically analysed. The carrier fluid is then pumped back to the seabed by means of a small centrifugal or triplex pump.

By this preferred development of the invention, a great deal of geological information is gained during upper hole drilling, which is not available with existing techniques. Since upper hole or top hole sections can be anything up to 4,000 feet in depth, during which time no formation cuttings and / or hydrocarbon analysis is possible with existing techniques, this is a significant advantage achieved by this preferred development of the invention.

Preferred embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a downwardly open gas collecting housing of a seabed gas collecting and monitoring device according to the invention:

Figure 2 is a vertical sectional view taken on the line A-A in Figure 1 :

Figure 3 is a schematic side view of the embodiment of gas collecting and monitoring device mounted on a usual guide base arranged on the seabed;

Figure 3a is a detail enlarged view of part of Figure 3. showing a gas detector device:

Figure 4 is a plan view corresponding to Figure 3; Figure 5 is a detail plan view of a further construction of housing:

Figure 6 is a section taken on section line B-B in Figure 5:

Figure 7 is a section taken on section line C-C in Figure 5 showing an alternative embodiment of gas detection device: and.

Figures 8a and 8b provide a schematic illustration of a preferred development of the device shown in Figures 1 to 4.

Referring first to Figures 1 to 4 of the drawings, a gas collecting and monitoring device according to the invention is designated generally by reference 10 and is intended to be used on a seabed in the region of entry of a borehole being formed downwardly from the seabed by a drill. A guide base 11 is shown in Figure 3, and may be of standard type, which is securely anchored to the seabed and which surrounds the borehole and serves to guide the rotating drill and following drill pipes. The guide base 11 has upwardly extending guide posts 12. and there will usually be guide lines 13 running between the rig (not shown) at the surface and the guide posts 12 in order to guide the movement of equipment between the surface and the guide base.

The embodiment of gas collecting and monitoring device 10 according to the invention comprises a downwardly open gas-collecting housing 14 made of flexible material such as neoprene and which is intended to be positioned above the entrance to the borehole, and to collect any gas which issues upwardly from the borehole. A hole 15 is arranged centrally in the upper surface of the housing 14 to allow a drill pipe to pass downwardly through the housing and into the borehole (see also Figure 8a).

A mounting frame or running frame 16 carries the housing 14 via mounting brackets 17 (see Figure 1). and is arranged to come into engagement with the guide posts 12 of the base frame 11 in order to mount the housing 14 above the borehole.

An outlet fitting designated generally by reference 18 is provided on the housing 14, and is arranged to direct any gas collected in the housing 14 laterally outwardly away from the borehole. An electrically operated gas detector device is arranged to monitor the presence of gas received by the housing 14 and passing to the outlet fitting 18. and in use will be arranged at a location sufficiently remote from the drill pipe that it can operate reliably to monitor the presence of gas, without any adverse interference from the rotating drill pipe.

In the illustrated arrangement, the gas detector device comprises a gas sampling probe 19 (see Figure 3), which is arranged in an outlet pipe 20 of the outlet fitting 18, and which responds to a change in electrical resistance across the probe caused by the presence of gas. in order to detect the presence and the level of gas.

Evidently, the volume of the outlet fitting is small relative to that of the housing 14. and therefore this provides concentration in the level of gas, whereby early warning indication can be given of the presence of gas. during drilling, even when the levels of gas are relatively low.

Thus, it is normally anticipated that gas levels steadily build-up in advance of the drill entering a gas pocket, and therefore advance warning can be given of any potentially hazardous condition, so that remedial action can be taken at the surface in good time.

A pod line 21 and counter weight 22 may be provided to control the mounting in position of the device.

Accordingly, the embodiment described above with reference to Figures 1 to 4 enables reliable monitoring of gas issuing at the seabed, and overcomes the following difficulties which are usually encountered:

1. the gas tends to rise as a plume from the well bore, dispersing as it rises:

2. the rotating drill pipe in the centre of the plume makes it exceedingly difficult for existing types of gas monitoring systems to function due to the "noise" created on electrical or other measuring systems.

The gas collection housing arrangement of the embodiment effectively collects and funnels any gas to a remote, insulated environment where gas can be detected using standard measuring systems without any masking effects. Also, by virtue of the large collecting area leading to a smaller conduit, the sensitivity for gas measurement is increased.

The gas collection housing is run on the guidelines connecting the surface rig to the permanent guide base, and consists of an inverted cap with an inclined top leading to a vent tube which could contain any suitable gas detection system.

Also, any gas coming from the well bore is channelled into a small neoprene conduit where its presence is detected by a set of resistivity plates connected to a resistivity meter on the rig. The housing effectively both amplifies the effect of the gas (due to the large collection area and the small conduit) and also, most importantly, isolates the measurement system from the masking effects of the rotating drill pipe.

Referring now to Figures 5 to 7. this shows in more detail a practical embodiment of construction of the gas- collecting housing, and with a modified mounting of the gas outlet fitting, and incorporating a further embodiment of gas detector device. The design of the housing is generally similar to that described above, and corresponding parts are designated by the same reference numerals, but with the addition of the letter a_.

Figure 5 is a plan view of gas collecting housing 14a having a central hole 15a to allow a drill pipe to pass downwardly through the housing and into the borehole, and which is defined by a neoprene sleeve 35 which is secured in position by means of a bushing 36 (e.g. of phosphor bronze) centrally on the upper part of housing 14a. Outlet fitting 18a is mounted on housing 14a as shown in Figures 5 and 6. and forms part of a rigid metallic casing 37 of housing 14a. Outlet fitting 18a conducts gas collected within housing 14a away to a remote gas detector, the construction of which is shown in more detail in Figure 7.

As can be seen in Figure 5. housing 14a has a hinge 38, the axis of which passes centrally through the axis of hole 15a.

Figure 7 is a section taken on C-C in Figure 5 i.e. along the axis of hinge 38. and shows central neoprene tube 35 mounted on rigid casing 37, and also shows outwardly extending outlet fitting 18a. which leads to a further embodiment of gas detector designated generally by reference 39. Gas detector 39 is located sufficiently remote from the rotating drill pipe as not to be adversely affected in its monitoring of the presence, or not. of gas in the detector. Gas detector 39 is an alternative embodiment to the electro- resistive gas monitor 19 shown in Figures 3 and 3a. and comprises an optical gas detector.

Gas detector 39 has an inlet pipe fitting 40 by which it can be fitted to the end of outlet fitting 18a, and a clear tube 41 extends centrally throughout the length of the gas detector 39. and is open at the inlet fitting 40, and closed at its opposite end 42. A protective shroud 43 is provided at the remote end of the gas detector 39, and in normal use. in the absence of any detection of gas. the interior of housing 14a. outlet pipe fitting 18a and the entire length of clear tube 41 will be filled with liquid. However, upon detection of gas issuing upwardly from the hole being drilled, this will be collected within the housing 14a, and then pass upwardly along outlet fitting 18a and then into tube 41. displacing the liquid away from closed end 42.

A series of optical sensors are arranged along the length of tube 41. and as the volume of gas increases, this is detected progressively by these optical sensors, giving appropriate warning signals. Thus, a first optical sensor 44 provides a first warning indication, which could be a preliminary warning signal transmitted to the surface installation, so that operating personnel at the surface can be put on a state of initial readiness to carry out any required corrective action at the surface installation. A second sensor 45 is arranged further along the length of the tube 41 towards the inlet end. and when this detects that the tube 41 is filled with gas from end 42 to the level monitored by sensor 45, this can give a more urgent warning to the surface of a potentially hazardous eruption of gas, so that remedial action can be taken.

Sensors 44 and 45 comprise annular arrangements of emitters and receivers, of which each emitter passes any suitable optical signal e.g. an infra red signal through the wall of the clear tube 41 and. when liquid is displaced by a gas bubble to uncover the respective receiver, the receiver can receive this signal and trigger a suitable warning signal, via microprocessor printed circuit board 46 and PSU / data output circuit board 47.

The electro-optic sensors arranged in gas detector 39 are housed within an annular chamber 48 defined between the outer surface of tube 41 and the inner wall of housing 49 of the detector 39. Chamber 48 is filled with suitable liquid to balance the pressures, and an electrical connector 50 enables suitable electrical output signals to be conveyed from the gas detector 39 to monitoring apparatus at the surface installation.

In a particularly preferred development of the device of the invention, the problem of seabed deposits of drill cuttings is addressed, in that with existing techniques of upper hole drilling without a marine riser, it is not possible to carry out regular monitoring of the cuttings to extract necessary geological data as drilling progresses. A further preferred embodiment will therefore now be described with reference to Figures 8a and 8b. in which Figure 8a shows the "sub-sea" arrangement and Figure 8b shows the "on rig" arrangement.

In the arrangement of Figure 8a. the gas collecting housing and its mounting above the borehole remains as previously described with reference to Figures 1 to 4, but the outlet fitting 18 of the housing 14 leads to a jet pump arrangement 23 which is operable to pump any gas, collected by housing 14 and passing along outlet fitting 18. up to the rig at the surface via a suitable flexible line 24. The jet pump 23 is powered by any suitable pressure fluid supplied to the pump via a pressure supply line 25 running from the rig to the pump. The action of the jet pump 23 is such as to create a sufficient pressure drop within the housing 14 in order to pull-in a sample of drilled formation cuttings which is also passed-up with the gas to the rig for routine analysis, though coarse mesh screen 26 over the entrance to fitting 18 can hold back large cuttings.

The monitoring device provided on the rig is shown schematically in Figure 8b. in which gas supplied to the surface with the power fluid and cuttings via line 24 is removed by a small de-gasser 27 and is then passed for analysis to a sensitive gas chromatograph of a mud-logging unit 28 which is connected to a rig alarm system 29. Cuttings 30 are removed by means of a sieve unit 31 and then physically analysed. The carrier fluid is then conveyed via return line 32 with header tank 33 to a small centrifugal or triplex pump 34, which pumps the carrier fluid back down supply line 25.

If gas is encountered, it is expected that gas levels would steadily increase as the drill bit nears the gas pocket. This will give advance warning. Alarm levels could be set above the background gas levels such that, if an alarm is triggered, the normal contingency plans for pulling the rig off location could be put into action.

Claims

CLAI MS

1. A gas collecting and monitoring device (10) which is intended to be used on a seabed in the region of entry of a borehole being formed downwardly from the seabed by a drill, there being a guide base (11) on the seabed around the borehole and serving to guide the rotating drill, and which guide base (11) has upwardly extending guides (12) and guidelines (13) running between the surface and the guides in order to guide the movement of equipment between the surface and the guide base (11). and said gas collecting and monitoring device (10) comprising: a downwardly open gas-collecting housing (14) which is intended to be positioned above the entrance to the borehole, and to collect any gas which issues upwardly from the borehole: a hole (15) arranged in the housing to allow a drill pipe to pass downwardly through the housing: a mounting frame (16) carrying the housing (14) and arranged to come into engagement with the base frame guides (12) in order to mount the housing (14) above the borehole: an outlet fitting (18) provided on the housing (14) and arranged to direct any gas collected in the housing (14) laterally outwardly away from the borehole; and. an electrically operated gas detector device (19, 39) for monitoring the presence of gas in the outlet fitting (18), the detector device being arranged in use at a location sufficiently remote from the drill pipe that it can operate reliably to monitor the presence of gas without being influenced by the drill pipe.

2. A gas collecting and monitoring device according to claim 1. in which the gas detector device (19) is mounted in outlet fitting (18) .

3. A gas collecting and monitoring device according to claim 1. in which the gas detector device (39) is coupled with an outlet end ^"of outlet fitting (18) to receive any gas collected in the housing (14) and passing upwardly into outlet fitting ( 18) .

4. A gas collecting and monitoring device according to claim 2. in which the gas detector device (19) is an electro- resistive device.

5. A gas collecting and monitoring device according to claim 3. in v/hich the gas detector (39) is an electro-optical detector device.

6. A gas collecting and monitoring device according to claim 5, in which the gas detector device (19) comprises a housing (47). a clear tube (41) extending through the housing (49) to define a chamber (48) between the outer wall of tube (41) and the housing (49). an inlet fitting (40) connectable to the outlet end of outlet fitting (18a), means (42. 43) closing the opposite end of tube (41) . and an electro-optical sensor arrangement (44 to 47) arranged in chamber (48) and operable to monitor the absence, or the presence of gas within tube (41 ) .

7. A gas collecting and monitoring device according to claim 6. in which the electro-optical sensor arrangement comprises a first electro-optical sensor (44) arranged to monitor a first level of gas received by the tube (41) , and a second optical sensor device (45) spaced from the first sensor (44) and arranged to monitor a second level of gas received by tube (41). and each operable to initiate respective warning signals.

8. A gas collecting and monitoring device according to claim 7, in which each optical sensor (44. 45) comprises an emitter and a receiver arranged at opposite sides of the clear tube (41), the arrangement being such that the presence of liquid in the tube (41) blocks transmission of an optical signal between each emitter and respective receiver, and the presence of gas allows the passage of the optical signal whereby an appropriate warning signal can be given.

9. A gas collecting and monitoring device according to claim 8, in which chamber (48) is an annular chamber which

^" can be filled with a sealed volume of pressure balancing 1 iquid .

10. A gas collecting and monitoring device according to any one of the preceding claims, in which the outlet fitting (18) leads to a jet pump (23) which is operable to pump any gas received by the housing (14⁾ to the surface via a delivery line (24) to run between device (10) and a surface installation, thereby to create a pressure drop within the housing (14) to pull-in a sample of drilled formation cuttings which may be generated during the drilling operation.

11. A gas collecting and monitoring device according to claim 10. including a supply line (25) to run between the surface installation and the device (10) to deliver a driving pressure medium to operate the jet pump (23).

12. A gas collecting and monitoring device according to claim 11. including a monitoring and sampling apparatus to be provided on the surface installation and connectable to the delivery line (24) and to the supply line (25). said apparatus including a gas separator (27) for separating gas from the driving pressure medium, a gas analyser (28) . an alarm system (29) responsive to a warning signal developed by the gas analyser (28). a "cuttings" separator (31). and a pump (34) for returning the driving pressure medium along supply line (25) to the jet pump (23).